I'm not 100% sure of the semantics of DWithin and Beyond, but I assume
that they test if two geometries are within or further than a given
distance from each other.
These seem like complementary functions, so I'm not sure why both are
provided - but perhaps it's awkward to express negation in Filter?
Anyway, GEOS, like JTS, provides the isWithinDistance method which
computes this predicate.
As a further note, the reason for having this as a separate predicate
rather than simply using distance() is that in theory isWithinDistance
can be optimized more easily and aggresively than distance().
Hope this helps... Martin
--
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022
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